An ultrasonic diagnostic apparatus using the pulse echo method of imaging the inside of a living body by transmitting/receiving pulse ultrasonic waves to/from the living body is widely used for medical diagnosis as well as X-ray CT and MRI. The maximum advantage of ultrasonic diagnosis which is not in the other image diagnostic modalities such as X-ray CT and MRI is high imaging speed that enables real-time image display. Specifically, the speed is as high as time resolution of human vision, that is, imaging speed at which an image can be updated approximately every 30 ms. Further, to observe the movement of the valve of the heart by low-speed reproduction, time resolution of acquiring an image every 15 ms can be even realized.
On the other hand, in the spatial resolution achieved by the pulse echo method, the distance resolution in the depth direction is obtained by resolution of time required for an ultrasonic pulse to travel to and back from a reflector. The propagation speed of an ultrasonic wave in a living body is 1500 m/s which is almost equal to that in water. Consequently, when the ultrasonic frequency is a few MHz or higher, the distance resolution of about 1 mm can be easily obtained by time resolution of about 1 μs.
On the other hand, the spatial resolution in the direction orthogonal to the depth resolution, that is, the azimuth resolution is obtained by focusing a transmission or reception wave. To obtain the azimuth resolution equal to or less than a few times as high as the wavelength of an ultrasonic wave, strong focusing to an extent that F number becomes almost 1 is necessary. As a result, the focal area depth corresponding to depth of field in the case of a camera becomes as low as a few wavelengths. It corresponds to about 1 μs of round-trip propagation time of an ultrasonic wave. By the dramatic advance of the high-speed electronic circuit technique of recent years, the reception focal length can be changed during the period in which an ultrasonic wave propagates in this distance. This is a so-called dynamic focus technique (refer to, for example, “New Medical Ultrasonics, Vol. 1, Basics of Medical Ultrasonic Waves, May 15, 2000, pp 40 to 41).